1. Field of the Invention
This invention generally relates to a non-polarizing Bragg-reflecting liquid crystal display which includes a FOFP located on the front surface of the reflective display.
2. Description of Related Art
Bragg-reflecting displays are well known in the art. FIGS. 1-5 show several types of conventional Bragg-reflecting liquid crystal displays (LCDs). For example, FIG. 1 shows a display that uses cholesteric LC materials, FIG. 2 shows a display that uses liquid crystals with polymer stabilized cholesteric textures (PSCT), FIG. 3 shows a display that uses liquid crystals with surface stabilized cholesteric textures (SSCT), FIG. 4 shows a display that uses polymer dispersed cholesteric liquid crystal (PDCLC) and FIG. 5 shows a display that uses holographically formed polymer dispersed liquid crystal (H-PDLC). A brief description of each of these five types of displays is provided below.
FIG. 1a shows a first substrate 10, a second substrate 20 and cholesteric liquid crystal materials located between the first and second substrates. In the off-state, a single domain reflects light with the approximate wavelength, xcex=nP, that satisfies the Bragg condition, where n is the average index of fraction and P is the pitch length associated with the chiral liquid crystal. The pitch length governs the selective wavelength or color to be reflected. All other wavelengths of light are transmitted. The off-state configuration of the LC is referred to as the planar texture as shown in FIG. 1a. As shown in FIG. 1b, upon application of an electric field by a voltage source 25, the pitch axes form an intermediate disorganized state known as the focal conic texture. This state is weakly scattering and the background (usually black) is easily visible. The focal conic state is metastable and may remain for hours before relaxing back to the planar texture (FIG. 1a). As shown in FIG. 1c, when a larger electric field is applied, all the cholesteric LC molecules align parallel to the field (for an LC material with positive dielectric anisotropy, +xcex94xcex5) and the display is transparent so that the background is observed. This is therefore a monochrome display that typically operates between a reflected color xcex and the color of the background which is usually a black absorber (not shown). The angular dependency of the display is strongly dictated by the Bragg condition, xcex=nP cos xcex8, where xcex8 is the angle between an observer and the normal to the substrate 10. Therefore, as the source of illumination and observer move off axis, the peak reflection shifts to shorter wavelengths.
The PSCT display shown in FIG. 2 operates in a similar manner to the display shown in FIG. 1, except a small amount of polymer forming network is added to stabilize the focal conic state indefinitely. As shown in FIG. 2a, the display operates according the Bragg condition, xcex=nP, in the off state. When a low electric field is applied as shown in FIG. 2b, the focal conic texture forms. However, the polymer network stabilizes the focal conic texture so that the electric field can be turned off and the focal conic texture remains indefinitely. Upon application of a larger electric field as shown in FIG. 2c, a completely aligned texture arises (for LC materials with positive dielectric anisotropy, +xcex94xcex5).
After the field is removed, the configuration relaxes back to the planar texture in FIG. 2b. This display is typically operated between the planar texture (FIG. 2a) and the focal conic texture (FIG. 2b) for color monochrome operation and bistable memory operation. The angular dependence of the display is also strongly dictated by the Bragg condition, xcex=nP cos xcex8, where xcex8 is the angle between the observer and the normal to the substrate 10.
The SSCT display shown in FIG. 3 operates under the same principles as the PSCT display except a random-type surface alignment is used instead of the polymer network. FIG. 3a shows the planar texture, FIG. 3b shows the focal conic texture and FIG. 3c shows the aligned texture. The random-type, non-rubbed surface alignment gives added stability to the focal conic texture (FIG. 3b) for bistable memory operation.
The PDCLC display shown in FIG. 4 also utilizes Bragg-reflection in a manner similar to that in FIGS. 1-3, except the LC configuration is different. The PDCLC employs droplets of cholesteric LC material dispersed in an isotropic polymer. The cholesteric LC material is of the negative dielectric type (xe2x88x92xcex94xcex5). In the off state shown in FIG. 4a, the stable concentric director configuration is nearly transparent. As shown in FIG. 4b, upon application of an electric field, the cholesteric LC molecules align perpendicular to the field direction because of their xe2x88x92xcex94xcex5 and form the planar texture within the droplets. Therefore, the display is reflecting in the field-on state. After the field is removed, the planar texture (FIG. 4b) reverts back to the concentric texture (FIG. 4a). The angular dependence of the display is also strongly dictated by the Bragg condition, xcex=nP cos xcex8.
The H-PDLC display shown in FIG. 5a uses optical interference techniques to phase separate the droplets of nematic LC and polymer into separate and distinct planes. This sets up a modulation in droplet densities, regions of droplets and regions of polymer. The resulting optical interference of this refractive index modulation is strongly dictated by the Braggs condition. The angular dependence of the display is also strongly dictated by the Bragg condition, xcex=nP cos xcex8. The H-PDLC display is advantageous because it can ideally reflect 100% of the incident illumination at the Bragg wavelength resulting in a brighter color display compared to those shown in FIGS. 1-4. As shown in FIG. 5b, upon application of an electric field, the refractive index modulation disappears if the ordinary index of refraction of the LC (no) matches that of the polymer (np) and all light is transmitted. After the electric field is turned off, the display relaxes back to the reflecting state shown in FIG. 5a. 
Fiber-optic faceplates (FOFPs) are also known in the art. U.S. Pat. Nos. 5,035,490 and 5,181,130 to Hubby, the subject matters of which are incorporated herein by reference, relate to reflective twisted nematic (TN) LCDs that utilize polarizers and FOFPs. The polarizers polarize the light passing through the LC cell. However, these displays are not Bragg-reflecting displays. Rather, incident illumination is polarized on input and passes through the entire LC cell and strikes a specular reflector that sends the light ray back through the LC cell, through at least one polarizer, used to analyze the polarization state exiting the LC cell, and out through the FOFP. The FOFP functions to expand the viewing angle and minimize the pixel xe2x80x9cshadowingxe2x80x9d of these traditional reflective twisted nematic LCDs.
Further, U.S. Pat. No. 5,442,467 and U.S. application Ser. Nos. 08/473,887 and 08/761,992, the subject matters of which are incorporated herein by reference, relate to direct-view transmissive color LCDs that utilize FOFPs. These direct view transmissive LCDs also utilize polarizers. The FOFP also acts as a front containing element adjacent to the LC layer. The FOFP provides azimuthal averaging of off-axis light. The azimuthal averaging properties of the FOFP result in symmetrical viewing cones, effectively averaging out the typical LCD anisotropy.
The front FOFP of the direct view display in U.S. application Ser. No. 08/761,992 includes an array of individual optical fibers that are fused together with an interstitial cladding material and then cut and polished to a desired thickness to form a plate. The creation of FOFPs with varying optical characteristics is well known in the art. The optical fibers are designed to transmit through total internal reflection light incident at controlled input or acceptance angles while rejecting or absorbing light incident at larger angles.
Prior applications of FOFPs on both reflective and transmissive LCDs have utilized twisted nematic LCDs, which rely on the principles of light polarization and polarization analysis for their operation. This makes their integration with FOFPs difficult and relatively inefficient since FOFPs do not preserve light polarization during internal reflection.
The present invention provides a non-polarizing, Bragg-reflecting LCD having a FOFP that reflects light in a symmetric output cone. The FOFP serves as the top substrate of the display to enhance light collection efficiency and viewing angle performance. The FOFP improves the overall off-specular viewing performance of the display by averaging azimuthal and declination angle components of both incident and reflected light. Monochrome reflective displays that operate on Bragg""s principal are severely limited in viewing angle because of the wavelength shift and luminance decay that occurs off the plane of incidence to the display. The FOFP stabilizes the chromaticity and effective reflected luminance for larger viewing angles. In addition, the FOFP may eliminate noticeable inhomogenities in reflective mode displays.
For purposes of the present invention, the term FOFP is interpreted in its broadest sense as any material which embodies the essential optical properties of a FOFP. Thus, the functioning of the present invention is not dependent upon the use of a fused plate of optical fibers but rather on any material layer, including a fused plate of optical fibers, which is capable of total internal reflection and rotational azimuthal and declination angle averaging. It should be apparent to those skilled in the art that these essential optical properties may be imparted to a range of materials, thus producing FOFP optical equivalents. These could include micro-machined or preformed glass or plastic substrates with a plurality of optical features, a variety of polymer networks containing a duality of materials with differing refractive indices or birefringence produced by physical alignment or stress, or any other approach able to result in a substrate containing a plurality of cylindrical features whose boundaries are defined by a discontinuity of refractive indices.
Other objects, advantages and salient features of the invention will become apparent from the following detailed description taken in conjunction with the annexed drawings, which disclose preferred embodiments of the invention.